This invention relates to the field of desktop publishing and, more particularly, relates to a software utility for converting full-color images to spot-color images including creating partitions that may be used to create printing plates and screens for offset and other types of printing.
Offset printing is a form of lithography in which ink is transferred from a plate containing the image to be printed on paper. The simplest and least expensive form of offset printing is black-and-white printing. The process begins by making a halftone partition (i.e., negative) of the original object in various tints of black. A halftone partition is formed by printing dots of ink of various sizes within a predefined area to achieve various tints of black. Next, a printing plate is made from the partition, usually by contact etching on a sensitized plate. Finally, the plate containing the image is placed in an offset printing press to produce a black-and-white reproduction of the image. A similar process is used to create screens for screen printing and other types of partitions for other printing processes.
Producing full-color reproductions of an image using offset and other printing processes is more complex and costly than simple black-and-white reproductions. To make full-color reproductions, a process known as color separation is employed. For example, color separation for an offset printing process consists of creating four separate printing plates, each plate for printing one of the four process colors, cyan, magenta, yellow, and black (CMYK). All colors within the image are then printed by blending layers of various sizes of halftone dots printed in these process colors to create the full-color image. Each plate is etched from a single-color negative known as a partition. To create the partitions, the image is divided into a partition for each of the four process colors by a process known as color separation. Color separation is typically performed by passing the image through a series of color filters and then through a screen to create the halftone partitions. Each halftone partition is then used to etch a positive image on an engraving plate so that each printing plate contains the halftone dots for printing only one process color.
Spot-color printing is an alternative printing process that can produce spot colors of higher quality and may often be significantly less expensive than full-color printing. In spot-color printing, one or more colored inks, other than the four process colors, are used to print the image. For example, in a COCA-COLA advertisement, it may be particularly important to print the COCO-COLA logo with the precise vibrant red that customers have come to associate with that product. Therefore, when printing the advertisement, it may be advantageous to print the logo using a red spot color. In addition, in some cases, the remainder of the image may be printed in tints of black, which requires two printing plates (i.e., spot-red and black) instead of the four plates required for full-color printing. Although this printing technique may reduce the number of colors, it can be less expensive and still meets the needs of users in certain circumstances. In general, spot-color printing is often an attractive alternative to increase the quality of a particular color in the printed images, and in some cases to reduce the cost of the printed images.
Typically, many users of desktop publishing computer programs design their publications in full-color without considering which printing process will be used to print the publication. When the time arrives to print the publication, however, the user may prefer spot-color printing over full-color printing for the reasons mentioned above. Unfortunately, at this stage of the printing cycle, it is often too late for the user to conveniently change the printing process. This is because in many conventional desktop publishing programs, the user typically has to recreate the publication from scratch in a spot-color process mode in order to create the required spot-color partitions. This can be a time consuming, inefficient, and frustrating process. Thus, there is a need for desktop publishing program that more easily convert images from full-color to spot-color for creating partitions that may be used to create printing plates and screens for offset and other types of printing.
Some desktop publishing programs have attempted to solve this problem by converting full-color to spot-color images in response to a user command. However, these programs employ an overly simplistic color conversion technique. Specifically, these programs identify objects in the image in which the associated colors are exactly the same as the spot color (i.e., color values are identical) or explicit tints of the spot color, and map those objects to a spot color partition. The program then maps all of the remaining unmapped elements in the image to the black partition. That is, only the exact spot-color, and explicit tints of the spot-color, are mapped to the spot-color partition. All remaining colors are mapped to the black partition (i.e., are printed as shades of gray). Many users find this color conversion technique inadequate because it maps the vast majority of the objects (i.e., all objects except those having the precise spot color in the original image, or an explicit tint of the spot-color) to the black partition, which is not what the user usually intends.
Another conversion technique allows the user to select more than one spot color. For example, the user could select two spot colors in addition to black. However, this conversion technique still maps only those objects that are exact matches of the two selected spot colors to a corresponding spot color partition, and maps the remaining objects, including those are tints of the spot colors, to the black partition. As a result, tints of the spot colors are still mapped to the black partition, and the vast majority of objects (i.e., all objects except those having the precise two spot colors in the original image) are still mapped to the black partition. Again, this is not what the user usually intends.
Still another conversion technique allows the user to select two spot colors to create an xe2x80x9cextended duotonexe2x80x9d image. This conversion technique combines the two spot-colors to represent the colors of the original image. As a result, most of the objects are printed as halftone blends of the two spot colors. That is, all of the objects that are not a tint of one of the spot colors in the original image are printed as halftone blends of the two spot colors. Many user find duotone printing unacceptable because blending the spot-color inks together reduces the visual impact of the individual spot-color inks. In other words, duotone printing diminishes the distinctiveness of the spot-color inks, which is often one of the important objectives of spot color printing. Instead, many users prefer to enhance the distinctiveness of the spot-color inks by avoiding the practice of blending the spot-color inks together.
Thus, there is a general need in the art for a more convenient and efficient method for converting images from full-color to spot-color. There is a further need for a method for converting images from full-color to spot-color without mapping tints of the spot to the black partition, without mapping most of the object to the black partition, and without blending the spot-color inks.
The present invention meets the above-described needs in a software utility for transforming an image from full-color to spot-color without mapping tints of the spot color to the black partition, and without blending the spot-color inks. Specifically, for a two-color print, all the tints of black are mapped to a black partition, and all remaining colors are mapped to a spot-color partition using a luminance matching technique. For a three color print, all tints of black are mapped to a black partition, all tints of a first spot color are mapped to a first spot color partition, and all remaining colors are mapped to a spot color partition using a luminance matching technique. This process does not diminish the distinctiveness of the spot-color process by blending spot color together. This process also maps the vast majority of the objects (i.e., all objects that are not a tint of black in the original image) to a spot-color partition.
Generally described, for a two-color print, a Spot Color Transformation (SCT) receives an image that contains a plurality of objects. Each object has an associated color that is defined by a set of color values. The SCT utility first maps each object that has an associated color that is substantially black or a tint of black to a black partition. The SCT utility then maps all remaining unmapped objects to a spot color partition using a luminance matching technique.
More specifically, the SCT utility calculates the luminance values of each unmapped object in the image. Luminance, which generally connotes the perceived brightness of the color, may be computed as a weighted average of the RGB color values for the color. Next, the SCT utility transforms the colors of the remaining unmapped objects to a percent tint of the spot color. The percent tint is based on the luminance value of the particular color relative to the lowest luminance value found in the image. That is, the SCT utility assigns a maximum percent tint of the spot color to the color in the original image having the lowest luminance value. The SCT utility then computes the percent tint for each remaining color as the quotient of the difference between the luminance value of the particular color and a maximum luminance value, and the difference between the lowest luminance value and the maximum luminance value. The SCT utility then maps each transformed object to a spot color partition, which may be used to create printing plates, screens, or other types of image positives for making copies of the original image.
The SCT utility may also produce three partitions for printing in two spot colors and black. For this type of three-color separation, the SCT utility maps all tints of black to a black partition, all tints of a first spot color to a first spot color partition, and all remaining colors to a spot color partition using the luminance matching technique described above.
The SCT utility may operate on images consisting of RGB values, CMYK values, or HSY values. Stated differently, the SCT utility may operate on images defined by color values in the color-domain consisting of primary (additive) color values or process (subtractive) color values, and may also operate on images defined by color values in the hue domain. For images defined by color values in the color-domain, the SCT utility calculates a luminance value for each unmapped object as a weighted sum of the color values for the object.
The invention also includes a method for making copies using the methods described above for the SCT utility and also creating copies based on the partitions produced by the SCT utility. For example, printing plates may be created from the partitions and used to print copies of the image on an offset printing press. Similarly, silk screens may be created from the partitions and used to print copies of the image on a screen printing machine. Other types of partitions and printing positives may also be created and used to make copies in accordance with the present invention. In sum, the invention includes an apparatus operable for carrying out the methods described above, and a computer-readable storage device storing a set of computer-executable instructions for performing those methods.